1. Field of the Invention
The present invention relates to crystal oscillators. More specifically, the present invention relates to high accuracy, low noise, digitally controlled crystal oscillators.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
A voltage controlled oscillator (VCO) is an oscillator having an output frequency that is a function of the input voltage. Voltage controlled crystal oscillators are used in many applications. For example, in a radar application, voltage controlled crystal oscillators are used to provide a variable frequency output signal and to convert radar target return signals to specific frequencies. Conversion of the radar target return signals to specific frequencies provides for optimum filtering and target detection. Thus, the more accurate the specific frequency setting by the voltage controlled crystal oscillator within the radar receiver, the less time is required for target acquisition.
In addition to frequency accuracy, phase noise is another significant parameter of voltage controlled oscillators. Phase noise is a measure of purity of the signal. Phase noise arises from random fluctuations in the frequency produced by the oscillator and manifests itself as an inaccuracy or jitter on the oscillator output signal frequency. Phase noise is often detected by a radar receiver to the exclusion of weak returns. As a result, weak signals are often not detected by the radar system. Thus, the lower the phase noise of the voltage controlled crystal oscillator, the higher the probability that the system will be able to detect, acquire and track a target.
Voltage controlled crystal oscillators typically utilize analog control signals. Unfortunately, crystal oscillators employing analog control signals offer limited output frequency accuracy and are beset with higher phase noise relative to more current approaches.
Another technique currently employed to provide digitally controlled oscillator signals is known as direct digital synthesis (DDS). The DDS technique overcomes many of the problems associated with voltage controlled crystal oscillators by utilizing a digital control signal. The digital control signal allows for accurate setting of the output signal frequency. Further, the phase noise generated by the DDS technique is lower than that generated by a typical voltage controlled crystal oscillator.
The DDS system includes a digital section, a digital-to-analog converter section and an output analog section. The digital information provided to the digital-to-analog section represents the instantaneous amplitude of an RF (radio frequency) waveform. The function of the digital section is to generate a sinusoidally oscillating digital number. First, a frequency number is digitally defined and then digitally integrated in a counter to provide instantaneous digital phase information. Finally, the digital phase information is converted to a digital amplitude word by a look-up table stored in a Read-Only-Memory (ROM).
Unfortunately, the DDS technique suffers from several undesirable limitations which include high component expense and complexity, high power consumption, and output signals having a high spurious content (spectral purity). Typically, it is desirable for the oscillator to generate a single frequency signal. A signal having a high spurious content is one that includes frequencies other than the desired frequency. The additional spurious signal frequencies are detected by the radar receiver as phantom targets. The radar receiver thereafter tracks the phantom targets. An output signal having a high spurious level is to be distinguished from an output signal having high phase noise. A high phase noise signal complicates the detection of weak signals, while a signal with a high spurious level results in an undesirable tracking of a phantom target. Both problems contribute to poor performance of the radar system.
Thus, there is a need in the art for an improvement in digitally controlled crystal oscillators employed in radar tracking systems.